What the heart wants: adaptive significance of cordate leaf morphology in Arnica (Asteraceae).

Large, thin, horizontally oriented leaves under forest canopies select for an increasingly central petiole insertion point to minimize leaf construction costs for a given leaf area, driving the development of cordate leaf bases. However, whether leaf morphology limits leaf orientation and thus species distributions in shaded forest understories has been studied in few taxa. We studied how the leaf inclination of basal leaves of 2 congeners, heartleaf arnica (Arnica cordifolia Hook.), which possesses cordate leaf bases, and broadleaf arnica (Arnica latifolia Bong.), which does not, varied along a canopy cover gradient from subalpine forests to meadows in the Greater Yellowstone Ecosystem, Wyoming, United States. Though individuals of both species were found across understory light environments, individuals of heartleaf arnica were capable of reaching lower leaf inclinations than individuals of broadleaf arnica in the shadiest environments, consistent with biomechanical limitations to leaf inclination in leaves lacking cordate bases. Indeed, cordateness (the depth of leaf lobing) in heartleaf arnica increased with greater canopy cover, driving concomitant decreases in petiole-leaf angles. Further ecophysiological study of these and other congeners with conspicuously variable leaf morphology may prove particularly insightful into the adaptive significance of both individual and suites of plant traits. [ABSTRACT FROM AUTHOR]